1. Field of the Invention
The present invention relates generally to a Wavelength Division Multiple Access (WDMA) free space broadcast technique for optical backplanes and interplanar communications, and more particularly pertains to a WDMA free space broadcast arrangement for optical backplanes and interplanar communications for providing free space optical interconnects between multiple circuit cards in a computer system or networking device which is compatible with existing electrical backplanes. With the present invention, current equipment can easily be upgraded in the field to take advantage of this new approach by simply replacing existing printed circuit boards, without requiring a complete redesign of the copper backplane.
2. Discussion of the Prior Art
Electrical interconnects are emerging as a bottleneck in the performance of large enterprise servers and supercomputers, as well as in data communication networks for metropolitan areas (50–100 km).
Future requirements for bandwidth in the backplanes of computers, networking equipment, digital cross-connect switches, routers and multiplexers are expected to require on the order of 5–10 Tbit per second throughput. This can be most readily achieved by using some form of optical interconnect. Various schemes have been proposed, including routing optical fiber across an electrical backplane or by using polymer-type materials to fabricate surface waveguides for optical signals. However, the approaches suggested to date are typically very costly to implement and face complex technical problems with alignment of the optical fibers, light sources and receivers (which are typically present in large arrays).
The use of VCSEL (vertical cavity surface-emitting lasers) lasers has further complicated this problem because these laser sources, although low cost and highly reliable, emit light perpendicular to the substrate; this requires some form of optical surface connection in an array form to accommodate VCSEL laser arrays and receiver arrays. Furthermore, existing optical bus proposals have not been widely implemented because they require an extensive redesign of the equipment backplane to convey optical signals from one location to another.
There is a need for an optical interconnect technology which addresses the use of VSCEL area array connections, is compatible with existing legacy card/board manufacturing processes, and does not require a redesign of the entire backplane of the networking equipment in order to achieve a higher I/O bandwidth. If such a solution were widely available, it would solve a bottleneck problem in the MAN and enable new types of optical interconnect solutions.
The explanations herein discuss both wavelength and frequency, which have a reciprocal relationship (λ=c/f, where c=speed of light), as is well known in the field of optics.
Wavelength Division Multiplexing (WDM) and Dense Wavelength Division Multiplexing (DWDM) are light-wave application technologies that enable multiple wavelengths (colors of light) to be paralleled into the same optical fiber or through free space with each wavelength potentially assigned its own data diagnostics. Currently, WDM and DWDM products combine many different data links over a single pair of optical fibers by re-modulating the data onto a set of lasers, which are tuned to a very specific wavelength (within 0.8 nm tolerance, following industry standards). On current products, up to 32 wavelengths of light can be combined over a single fiber link with more wavelengths contemplated for future applications. The wavelengths are combined by passing light through a series of thin film interference filters, which consist of multilayer coatings on a glass substrate, pigtailed with optical fibers. The filters combine multiple wavelengths into a single fiber path, and also separate them again at the far end of the multiplexed link. Filters may also be used at intermediate points to add or drop wavelength channels from the optical network.
Ideally, a WDM laser would produce a very narrow linewidth spectrum consisting of only a single wavelength, and an ideal filter would have a square bandpass characteristic of about 0.4 nm width, for example, in the frequency domain. In practice, however, every laser has a finite spectral width, which is a Gaussian spread about 1 to 3 nm wide, for example, and all real filters have a Gaussian bandpass function. It is therefore desirable to align the laser center wavelength with the center of the filter passband to facilitate the reduction of crosstalk between wavelengths, since the spacing between WDM wavelengths are so narrow. In commercial systems used today, however, it is very difficult to perform this alignment—lasers and filters are made by different companies, and it is both difficult and expensive to craft precision tuned optical components. As a result, the systems in use today are far from optimal; optical losses in a WDM filter can be as high as 4 db due to misalignment with the laser center wavelength (the laser's optical power is lost if it cannot pass through the filter). This has a serious impact on optical link budgets and supported distances, especially since many filters must be cascaded together in series (up to 8 filters in current designs, possibly more in the future). If every filter was operating at its worst case condition (worst loss), it would not be possible to build a practical system. Furthermore, the laser center wavelengths drift with voltage, temperature, and aging over their lifetime, and the filter characteristics may also change with temperature and age. The laser center wavelength and filter bandwidth may also be polarization dependent. This problem places a fundamental limit on the design of future WDM networking systems.
A second, related problem results from the fact that direct current modulation of data onto a semiconductor laser diode causes two effects, which may induce rapid shifts in the center wavelength of the laser immediately after the onset of the laser pulse. These are (1) frequency chirp and (2) relaxation oscillations. Both effects are more pronounced at higher laser output powers and drive voltages, or at higher modulation bit rates. Not only can these effects cause laser center wavelengths to change rapidly and unpredictably, they also cause a broadening of the laser linewidth, which can be a source of loss when interacting with optical filters or may cause optical crosstalk. Avoiding these two effects requires either non-standard, expensive lasers, external modulators (which are lossy and add cost), or driving the laser at less than its maximum power capacity (which reduces the link budget and distance). Lowering the data modulation rate may also help, but is often not an option in multi-gigabit laser links.
It would thus be highly desirable to provide a stable, optimal alignment between a laser center wavelength and the center of a Gaussian bandpass filter in order to optimize power transmission through such fiber optic systems or through free space and reduce optical crosstalk interference in optical networks.